Add a Delegate to JavaScriptAppModalDialog.

      Done a couples of things here:

- Renamed JavaScriptMessageBoxClient to JavaScriptMessageBoxDelegate (since this is a common pattern in chrome and helps the reader to find out where the message box goes).
- Moved that class into js_modal_dialog.h file.
- Deleted jsmessage_box_client.h

It is a following to http://codereview.chromium.org/3384036.

BUG=54988
TEST=trybots

Review URL: http://codereview.chromium.org/3576003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@61309 0039d316-1c4b-4281-b951-d872f2087c98